1. Technical Field
The present invention relates to a zoom dial mechanism which sets a zooming operation of power zoom lens, and more particularly to a zoom dial mechanism suitable for use with a visual presenter provided with a power zoom lens.
2. Description of the Related Art
A visual presenter comprises a base, a pillar standing on the base and a camera head provided on a distal end of the pillar. Material such as document, model or the like is imaged by the camera head. Images thus obtained by the camera head are reproduced by a monitor television or projected on a screen by a video projector.
Japanese patent application publication, JP-A-2007-194884, discloses one type of visual presenter as described above. More specifically, the visual presenter 100 comprises a base 101, a pillar 102 standing on the base 101, and a camera head 103 mounted on a distal end of the pillar 102, as shown in FIG. 8. The pillar 102 of the visual presenter 100 includes an upright portion 102a upstanding on the base 101 and a horizontal portion 102b extending horizontally forward continuously from the upright portion 102a. The horizontal portion 102b has a distal end on which a cylindrical camera head 103 is mounted. A power zoom lens 104 is incorporated on the underside of a distal end of the camera head 103. Furthermore, a zoom dial mechanism 200 is incorporated in a proximal end of the camera head 103. In the camera head 103 are built a zoom mechanism of the power zoom lens 104, an automatic focusing mechanism and a control circuit for these mechanisms.
In the visual presenter as shown in FIG. 8, material is placed on an installation surface so as to be located near a front of the base 101. The zoom dial 201 is manually turned so that a field angle of the power zoom lens 104 is adjusted, and a central push knob 202 of the zoom dial 201 is pushed so that the automatic focusing mechanism is activated, whereupon the material is imaged by the camera head 103.
The aforesaid zoom dial mechanism 200 comprises a base plate 203, a zoom dial 201 and an automatic focusing push knob 202 as shown in FIG. 9. The zoom dial 201 is provided in a central hole 203a (see FIG. 10) of the base plate 203 so as to be turnable clockwise and counterclockwise. The zoom dial 201 sets a zooming operation of the power zoom lens 104 according to the direction of turn and an amount of turn thereof. The automatic focusing push knob 202 is mounted in a central hole 201a of the zoom dial 201 so as to be movable forward and rearward relative to the central hole 201a. 
In the above-described zoom dial mechanism, the base plate 203 has three protrusions 203b, 203c and 203d formed around the central hole 203a thereof as shown in FIG. 10. A torsion spring 204 is mounted on the protrusions 203b, 203c and 203d to urge the zoom dial 201 toward a rotational neutral position. The torsion spring 204 is disposed so as to encompass the central hole 203a of the base plate 203. The torsion spring 204 has one end 204a which locks a first rib 201b provided on the zoom dial 201 and the other end 204b which locks a second rib 201c of the zoom dial 201. When the zoom dial 201 is turned clockwise, the one end 204a of the torsion spring 204 expands clockwise by the first rib 201b. When the user detaches his or her fingers from the zoom dial 201 to stop dial operation, the end 204a of the expanded torsion spring 204 contracts such that the zoom dial 201 returns to the neutral position. When the zoom dial 201 is turned counterclockwise, the other end 204c of the torsion spring 201 expands counterclockwise by the second rib 201c. When the user stops dial operation, the zoom dial 201 returns to the neutral position.
The torsion spring 204 has a larger diameter than the central hole 203a of the base plate 203 in the above-described conventional zoom dial mechanism 200. The torsion spring 204 is mounted on the base plate 203 so as to surround the central hole 203a. Accordingly, the torsion spring 204 necessitates a sufficient space around the central hole 203a to be mounted on the base plate 203. This increases the size of the base plate 203, rendering size reduction of the dial mechanism 200 difficult.
Furthermore, the pin-like ends 204a and 204b of the torsion spring 204 and the ribs 201b and 201c easily disengage from each other respectively. This not only causes failure in the zoom dial mechanism 200 but also necessitates careful assembly of the base plate 203 and the zoom dial 201, the latter of which reduces the assembling efficiency.